From: Rich Felker <dalias@libc.org>
To: musl@lists.openwall.com
Subject: Re: [PATCH] Add stdatomic.h for clang>=3.1 and gcc>=4.1
Date: Sat, 22 Nov 2014 20:43:54 -0500 [thread overview]
Message-ID: <20141123014354.GF29621@brightrain.aerifal.cx> (raw)
In-Reply-To: <1416706295.16006.354.camel@eris.loria.fr>
On Sun, Nov 23, 2014 at 02:31:35AM +0100, Jens Gustedt wrote:
> Hi Rich,
>
> Am Samstag, den 22.11.2014, 18:30 -0500 schrieb Rich Felker:
> > atomic_flag is not viable for this because it does not have a
> > wait/wake mechanism. You'd be spinning, which means in processes with
> > different priorities involved, you could easily get deadlock if the
> > lower-priority thread got suspended while holding the lock. You really
> > do need mutexes.
>
> I am probably still too much thinking in C11, only, which doesn't have
> the notion of priorities.
>
> Actually, I think a specially cooked synchronization tool would be
> better. Something that combines an atomic pointer (to point to the
> object) with a futex living on it for the waiting. This would probably
> be a bit more challenging to implement, but here we really have an
> interest to have the fast path really fast, just one CAS of the
> pointer.
I don't get what you mean. To access an atomic object larger than the
hardware supports, you have to hold a lock for the whole interval of
reading/writing. This is O(n) in the size of the object. I don't see
where your ideas about pointers and CAS are coming in.
> > > What has all of this to do with VLA? I am lost.
> >
> > The operands of __typeof__ and sizeof get evaluated when they have VLA
> > type. I think this is the problem.
>
> ah, ok
>
> No, this isn't a problem, I think. Arrays aren't allowed to be subject
> of an _Atomic qualification (arrays are never qualified
> themselves). For _Atomic type, the standard explicitly excludes
> arrays. So arrays in general and VLA in particular should never be
> passed as such into any of these generic functions, only pointers to
> atomic objects can.
Is a pointer to a variably modified type considered variably modified?
If so maybe these are affected too...
> > > > I have changed it to be an atomic_bool in a struct as both GCC and Clang
> > > > has it in a struct. Presumably to separate it from the generic _Atomic
> > > > stuff.
> > >
> > > Again, since we want to have ABI compatibility, it is not your choice
> > > to make. You'd simply have to stick to the choice that gcc made. So
> > > you have to copy the declaration of the struct, including all the
> > > ifdef fuzz.
> >
> > I'd have to look at it again, but IIRC only one case of the #ifdef
> > mess was actually possible. The others were for hypothetical archs
> > without real atomics which we can't support anyway.
>
> We should have it as a struct, if the implementations have it like
> that, I think:
>
> - It should have same alignment properties for ABI compatibility.
> - It should lead to the same typename when included in C++.
Yes.
> The ifdef is a single one to switch between _Bool or unsigned char or
> so.
Yes, but I think the #ifdef always comes out one way anyway, though I
don't remember which one and don't have the file in front of me.
Rich
next prev parent reply other threads:[~2014-11-23 1:43 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-09 12:53 Joakim Sindholt
2014-11-09 17:11 ` Jens Gustedt
2014-11-22 20:52 ` Joakim Sindholt
2014-11-22 23:09 ` Jens Gustedt
2014-11-22 23:30 ` Rich Felker
2014-11-23 1:31 ` Jens Gustedt
2014-11-23 1:43 ` Rich Felker [this message]
2014-11-23 1:47 ` Joakim Sindholt
2014-11-23 2:42 ` Rich Felker
2014-11-23 9:43 ` Jens Gustedt
2014-11-23 15:21 ` Rich Felker
2014-11-23 16:29 ` Jens Gustedt
2014-11-23 16:38 ` Rich Felker
2014-11-23 17:05 ` Jens Gustedt
2014-11-23 17:29 ` stephen Turner
2014-11-23 19:38 ` Rich Felker
2014-11-23 8:49 ` Jens Gustedt
2014-11-23 15:06 ` Rich Felker
2014-11-23 16:18 ` Jens Gustedt
2014-11-23 16:37 ` Rich Felker
2014-11-23 18:01 ` Jens Gustedt
2014-11-23 19:39 ` Rich Felker
2014-11-23 23:30 ` Jens Gustedt
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